1. Field of the Invention
The present invention generally relates to charged particle beam systems and, more particularly, to ion beam systems for depositing material on a semiconductor structure during manufacture of an electronic device, especially in providing epitaxial growth of layers thereof.
2. Description of the Prior Art
The manipulation of beams of charged particles with electrical and magnetic fields has long been known and many devices exploiting this effect have been developed. For example, cathode ray tubes in televisions and oscilloscopes manipulate an electron beam to produce visually perceptible images. Electron beam lithography is also used in the production of highly accurate patterned areas in the manufacture of very large scale integrated circuits (VLSI). It is also known to produce and manipulate beams of other kinds of charged particles, such as in ion beam devices. Such ion beam devices have been used to advantage in certain aspects semiconductor device manufacture, such as in impurity implantation.
Impurity implantation by means of an ion beam is desirable for a number of reasons. The ion beam current and implantation energy can also be very accurately controlled to provide extremely accurate concentrations and distributions of impurities and implantation depths. Such ion implantation processes can also be carried out at low temperatures, allowing the use of low temperature masking materials.
Moreover, the mass of the ion in relation to the charge thereon affects the degree to which it is accelerated both axially and transversely by an electrostatic or magnetic field. Therefore, the beam which reaches a desired area of a chip can be made very pure since ions of differing molecular weight will be deflected to positions away from the beam and implantation of other than desired materials can be avoided. This feature of the ion beam optics of ion beam devices is known as mass analysis and is typically carried out by deflecting the beam through an arc and using an exit aperture of a size which will effectively separate ions of different molecular weight.
Such implantation processes use kinetic effects and are done at high energy to implant the ions within the body of a semiconductor material. More recently, efforts have been made to use an ion beam Process for purposes which require deposition on the surface of a target material, such as for welding. As can be readily understood, a deposition process produced from an ion beam would require the energy of the ion particles to be very much lower than the energies at which implantation is performed. Such reduced energies of the ions cause some difficulties to be encountered in maintaining convergence of the ion beam due to the mutual repulsion of ions bearing a like charge. However, in such an application, the need for high beam current is not necessary because the amount of material is typically small.
The formation of monocrystalline epitaxial layers of a semiconductor material, particularly with conductivity determining impurities, is often necessary in the manufacture of various types of semiconductor devices. This process is often carried out through vapor phase deposition at very high temperatures of approximately 1100.degree.-1200.degree. C. With a few exceptions, such as P-doped and intrinsic silicon, good quality monocrystalline deposition is difficult below about 1000.degree. C. This high temperature requirement for forming a monocrystalline epitaxial layer therefore has the drawback that, particularly if other doped structures have previously been formed, out-gassing effects and/or out-diffusion between regions may occur. In device design, compensation for such effects is often difficult or impossible and can also limit the minimum dimension of conductivity region in the device for a particular manufacturing yield since impurity out-diffusion distances can easily dominate an epitaxial layer which in thinner than about 2 microns or a region of similar lateral dimension. Such out-diffusion due to the high temperature process also results in dopant distribution being less than fully controllable, even when ion implantation is subsequently used to add impurities to the monocrystalline epitaxial layer.
It should also be noted that ion implantation, by itself, does not completely eliminate the need for a high temperature process even though ion implantation can be carried out at low temperatures since the ion implantation process causes damage to the crystal lattice structure and annealing is often necessary to repair the damage before further processing can be carried out.
The use of an ion beam to provide a low temperature process for producing a monocrystalline epitaxial layer has been achieved and is disclosed in detail in Keller et al U.S. Pat. Nos. 4,151,420 and 4,179,312, assigned to the assignee of the present invention and hereby fully incorporated by reference. These techniques are characterized by the use of multi-aperture sources to obtain high ion beam current. Such multi-aperture sources produce a broad beam and it can be readily understood that a significant amount of ion beam current is lost at the mass analysis aperture if good separation of ion masses is to be obtained, even though condensing lenses are used for each of the superimposed beams.
These techniques achieved a relatively high beam current at the target at reasonably low energies of about 500 eV. However, these currents were spread over a relatively large area of the target (e.g. a beam diameter of about 15 cm). Thus, a beam current of about 1 ma/cm.sup.2 resulted in a rate of material deposition which limited the throughput of the process. Also, by using such a large beam diameter, the epitaxial growth process was limited to performance of the process over the entire wafer and selective epitaxial growth could not even be limited to the actual chip areas, wasting beam current directed to areas of the wafer between chips.
It has also been found, by the inventors herein, that even lower ion energies are desirable for epitaxial growth during the manufacture of a semiconductor device or other object, such as a mask or calibration grid. For instance, implantation may be performed at a typical energy of approximately 20 Kev, whereas, it has been found, by the inventors herein, that energies of 2 Kev or less are required for epitaxial growth and even lower values are desirable. While the arrangements of the above-incorporated Patents achieved energies of about 0.5 KeV at the target, even faster epitaxial growth can be achieved at energies of 50-300 eV. It has also been found that, for several reasons discussed in more detail below, energies of about 5 KeV are desirable for good performance of mass analysis where epitaxial growth consists of a material which may contain a plurality of elements (e.g. silicon and an impurity element such as boron and arsenic, depending on the conductivity type desired) and which must be deposited simultaneously at coincident locations to assure homogeneity in the epitaxial growth. Such homogeneity also requires that the ions of the different materials reach the target at the same impingement angle, preferably perpendicular to the target, in order to avoid a differential distribution of the elements in the direction of epitaxial growth, particularly if the beam is to be scanned over the region where epitaxial growth is desired. It is also necessary to maintain good beam convergence to assure homogeneity of the epitaxial growth.
Although the arrangements of the above-incorporated patents utilize deceleration lenses, such a differential between mass analysis energies and deposition energies is difficult to achieve consistent with high beam current at the target. Other conflicting requirements also exist in processes for achieving epitaxial crystal growth with ion beam devices. Specifically, it is desirable to perform the process at high vacuum to minimize the possibility of contamination of the deposition and to maintain beam current which would otherwise be reduced due to charge exchange between ions and molecules of gas which may be present. If the charge is removed from an accelerated ion, no further mass analysis can be performed to guide it and maintain it within the beam, thus reducing beam current. To avoid contamination, an atmosphere of a noble gas such as neon and silane (SiH.sub.4) is typically used. A high vacuum is also used to reduce reduction of beam current by charge exchange.
Since ions carry the same positive charge, they will mutually repel each other unless oppositely charged particles are available to neutralize the space charge of the beam. At particle energies of above 10 Kev, even at high vacuum, the beam energy imparted to the extremely low pressure atmosphere within an ion beam device will produce a plasma which will provide substantially full space charge neutralization at high vacuum. However, when the ion beam energy is reduced to 5 KeV or less, it has been found by the inventors that a lower vacuum atmosphere of about 1.times.10.sup.-4 Torr is required to neutralize the space charge of the beam. Even this low pressure severely reduces beam current due to charge exchange. Alternatively, if the space charge is less fully neutralized, beam current is lost during mass analysis due to the interfering effect of the mutual repulsion between ions, particularly if the beam is focussed or concentrated or current density otherwise increased, as the inventors herein have found to be desirable to enhance mass analysis.
It is also important to note that electrostatic deceleration lens arrangements, even when operated at fairly low voltages to reduce the particle energies from about 1-10 KeV to about 0.5 KeV, causes the beam to diverge. The divergence of the beam will also be increased since the space charge neutralizing particles must be removed from the ion beam prior to deceleration. Therefore, the mutual repulsion between ions will be great since the particle beam must be focussed at virtually the same point along the beam path that deceleration is desired. It is therefore particularly desirable to keep the beam energy low in order to reduce the operating voltage necessary for the electrostatic deceleration lens for minimization of beam divergence, especially at high current densities. However, this severely conflicts with the performance of mass analysis at high current densities and low energies as pointed out above.
It should also be noted that the deceleration lens is electrostatic and typically takes the form of an aperture of some type. Additionally, as disclosed in the above-incorporated patents, the beam is substantially collimated by a mass separator plate. While the mass separator plate provides the function of increasing the purity of the deposited material, it makes the focussing of the beams of ions of different elements especially critical if it is not to severely reduce beam current or alter the relative amounts of each type of ion deposited, particularly where broad multi-aperture ion sources are used to obtain high currents. This is also true of the deceleration lens and the beam must be accurately collimated to avoid introducing distribution differentials across the beam pattern at the target.